Isocyanates and isocyanate mixtures are produced according to the prior art by phosgenation of the corresponding amines. For polyurethane foams, for example, di- or polyfunctional aromatic isocyanates of the diphenylmethane series (MDI) are employed. As a result of the production process, after phosgenation and subsequent work-up (removal of the solvent; removal of monomeric MDI) dark-colored products are often obtained, which in turn give polyurethane foams with a yellowish discoloration or other polyurethane (PUR) materials that are also discolored. This is undesirable, because this coloring adversely affects the overall visual impression and allows slight inhomogeneities to become evident, e.g. as streaks in the foams obtained. Light-colored isocyanates, or isocyanates that contain a reduced quantity of coloring components, are therefore preferred as raw materials.
There has therefore been no lack of attempts to obtain isocyanates, and particularly the di- and polyisocyanates of the diphenylmethane series (MDI), with a light color. Numerous methods for the empirical color lightening of MDI are known. However, the nature of the problematic coloring substances has hitherto been clarified only inadequately.
The previously known processes can be divided into four groups:
1. Processes in which the di- and polyamines of the diphenylmethane series (MDA) used as starting material have been subjected to a treatment and/or purification:
EP-A 0 546 398 describes a process for the production of MDI in which the MDA used as starting material is acidified prior to phosgenation.
EP-A 0 446 781 relates to a process for the production of MDI in which the MDA is first treated with hydrogen and subsequently subjected to a phosgenation, a relatively light-colored MDI being obtained.
The above-mentioned processes give only a slight improvement in the color, as the coloring substances in the MDI have been found on the basis of experience not only to consist of certain by-products from MDA production but also to result from color precursors which are formed by side-reactions during the phosgenation.
2. Process engineering solutions in the phosgenation process:
U.S. Pat. No. 5,364,958 relates to a process for the production of isocyanates in which, after the phosgenation, the phosgene is removed completely at low temperature and the isocyanate is subsequently treated with HCl gas at elevated temperatures.
DE-A-1981 7691 describes a process for the production of MDI having a reduced content of chlorinated by-products and a reduced iodine color value by adherence to defined parameters in the phosgenation reaction. In particular, adherence to particular phosgene/HCl ratios in the reaction step is required. This process has the disadvantage that a variation of the parameters in the phosgenation is made difficult and the quality of the phosgenation is very sensitive as a result. In addition, the lack of flexibility in the parameters in the phosgenation makes the phosgenation very difficult to carry out in practice and requires a high engineering effort.
Another way of improving the color of isocyanates is, according to EP-B 1 187 808, the use of phosgene with low bromine and/or iodine contents.
Although processes of the type mentioned attempt to remove the components causing the discoloration at the correct point, they are not efficient enough, both because of their high technical requirements and also in terms of their color-lightening effect, given that only slight degradation of color precursors occurs owing to incomplete chemical reactions.
3. Addition of color-lightening additives to the crude isocyanate product after the phosgenation and before the work-up, e.g. by distillation:
EP-A 0 581 100 relates to a process for the production of isocyanates in which a chemical reducing agent is added after the phosgenation and before the removal of solvent, which according to this document also gives light-colored products.
According to U.S. Pat. No. 4,465,639, water is added to the crude product obtained after the phosgenation in order to lighten its color. EP-A 538 500, EP-A 0 445 602 and EP-A 0 467 125 describe the addition of carboxylic acids, alkanols or polyether polyols after the phosgenation for the same purpose.
Although the methods of lightening the color described above are efficient, they have disadvantages in that the additives not only lighten the color but also undergo reactions with the isocyanates obtained as product, generally resulting, for example, in an undesirable reduction in the isocyanate content. In addition, there is the risk of formation of undesirable by-products in the MDI.
4. After-treatment of the end product:
EP-A 0 133 538 describes the purification of isocyanates by extraction, giving fractions of a light-colored MDI.
EP-A 0 561 225 relates to a process for the production of isocyanates or isocyanate mixtures which, according to this document, contain no coloring components, wherein the isocyanates are subjected to a hydrogen treatment at a pressure of 1 to 150 bar and a temperature of 100 to 180° C. after the phosgenation of the corresponding amines. According to the examples described there, isocyanate end products are hydrogenated as such or in the form of their solutions in suitable solvents.
These color-improving after-treatments of the isocyanate end products after complete removal of the solvent at elevated temperature are likewise not very efficient, because the high temperatures occurring during the work-up, in particular during the distillation of the solvent and optionally of monomeric MDI (diisocyanates), have already resulted in the formation of stable coloring substances which can be chemically degraded only with difficulty.